1. Field of the Invention
The present invention relates to methods of manufacturing electrical parts, and particularly to manufacturing methods with which contact resistance on the surface of an electrical part can be decreased.
2. Description of the Background Art
In the past, lithography using ultraviolet irradiation was used for making fine electrical parts consisting of metals such as Ni (nickel) and Ni alloy with electroforming. However, in recent years, increasing need of electrical parts having higher precision and higher aspect ratio has attracted attention to the so-called LIGA (Lithographie Galvanoformung Abformung) process, in which lithography, plating such as electroforming, and molding are combined. It is possible to make electrical parts having sizes of micron order by the LIGA process, particularly by X-ray lithography using synchrotron radiation (SR).
An example of the methods of manufacturing electrical parts by the LIGA process using the X-ray lithography will be described below.
First, a photoresist is applied over the whole area of a surface of a conductive substrate made of Ni or the like. Subsequently, a mask in which patterning is provided in a predetermined shape is placed on the surface of the photoresist and SR light is irradiated from a position above the mask. Consequently, by removing the photoresist at the parts where SR light has been irradiated, the photoresist is patterned according to the predetermined shape, whereby a master die is formed. Thus, the X-ray lithography is accomplished. Subsequently, by performing electroforming using the master die, a metal layer is deposited on the surface of the substrate which is exposed without the photoresist being formed. In this case, the electroforming using the master die is performed by immersing the master die as a cathode in an electroforming bath to which metal for forming a metal layer is added, and by applying an electric current between the master die and an anode which is immersed separately apart from the master die in the electroforming bath. Then, the metal layer is partly removed by polishing the portion excessively deposited due to electroforming. (For example, refer to Application of Nickel Electro-forming for Micromachining by Manabu Yasui, Yasuo Hirabayashi, and Hiroyuki Fujita, “Hyomen Gijutu” 2001, Vol. 52, No. 11, pp.734-735.) (Note: The literal translation of “Hyomen Gijutu” means “surface technology.”) Thereafter, the photoresist is removed by plasma ashing using a mixed gas of carbon tetrafluoride (CF4) and oxygen (O2), and the metal layer is removed from the substrate such that an electrical part is completed.
However, regarding an electrical part made by such a method as described above, there was the problem that the contact resistance of the surface thereof was high. As a result of earnest study about this problem, the present inventors have found that a metal oxide film is formed on the surface of the metal layer where the excessive portion thereof has been removed by above-mentioned polishing and that, during plasma ashing of the photoresist, the metal oxide film (MOx; M represents metal, x is an integer) causes a reaction with hydrogen fluoride (HF), which is a decomposition product of ashing gas, such that MOx+2xHF→MF2x+xH2O, whereby a metal fluoride film, which is represented by the chemical formula of MF2x, is formed on the surface of the metal layer. Since the metal fluoride film is so compact and rigid that the removal thereof is difficult, it is desired to establish a method of manufacturing electrical parts in which no metal fluoride film is formed on the surfaces of the electrical parts.